Recently, as accompanied by the developments of portable electronic instruments, such as notebook personal computers and cellular phones, and the practical applications of electric automobiles, small-sized, lightweight and high-capacity secondary batteries have been needed. As a high-capacity secondary battery meeting the desires, lithium-ion secondary batteries have been developed.
As a positive-electrode active material for lithium-ion secondary battery, lithium metallic oxides, such as LiNi0.5Co0.2Mn0.3O2 and LiMn2O4, are given, for instance. Whereas the lithium metallic oxides have high energy densities, the lithium metallic oxides are of low thermal stability.
Meanwhile, another positive-electrode active material for lithium-ion secondary battery, an olivine-type phosphate compound, such as LiFePO4, is given. The olivine-type phosphate compound is less likely to release oxygen even when being heated to a high temperature, and is superior in the thermal stability (see patent application Publication Nos. 1 through 4).
Hence, patent application Publication No. 5 proposes to mix a lithium metallic oxide with an olivine-type phosphate compound in an appropriate mass ratio to use as a positive-electrode active material.